This invention relates to a device to measure mercury and other metals in flue gas from municipal waste incinerators, electric power utility plants, and manufacturing plants at concentrations as low as 10 parts per trillion (0.1 micrograms/mxe2x88x923).
The 1990 Clean Air Act Amendments and other legislation have raised the concerns over low trace concentrations of metals, especially mercury, in flue gas from electrical generation plants, municipal waste incinerators, and heavy industry. Monitoring will be used to determine the extent of the environmental impact and offers the possibility of empirically minimizing it. In particular, mercury has been found in significant quantities in the lakes and streams of the mid-west of the United States. The source of this mercury has been found to be from the air emissions from large stationary combustion systems. Technology to monitor the levels of mercury may be required by regulations and by technology to control emission levels. The mercury levels at incinerators will be in the range of 0.1-10 parts per billion. However, the levels at electric power utility plants are only 0.03-1.0 parts per billion. For an instrument to measure the lower levels of mercury, for example, it must have a minimum sensitivity on the order of 10 parts per trillion. Furthermore, the instrument must make measurements on flue gas that contains many different molecular gas species that can compromise the measurement of the trace metals, especially mercury.
One approach to measuring metals in gas streams uses plasma emission spectroscopy. U.S. Pat. Nos. 5,479,254 and 5,671,045 describe a device using high-powered (300 Watts) and high-pressure microwave plasma emission spectroscopy to measure metals in gas streams. The plasma source is continuous in operation. The device uses a shorted-waveguide as the plasma source and it is inserted in the gas stream. A microwave tuner is used to couple the high-powered microwaves to the shorted-waveguide. A high-resolution spectrometer (0.01 nm) is used to make the measurements. In U.S. Pat. No. 5,671,045 the device is modified for operation in harsh gas and high-temperature environment. There are no provisions for reducing competing emission from flue gas components nor enhancing the metals emission. The high-pressure operation requires the high microwave power to sustain the plasma discharge. There is no disclosure of measuring mercury. There are examples of measuring magnesium, chromium, and iron in a high temperature furnace as well as the laboratory.
U.S. Pat. No. 5,909,277 represents an improvement to the two patents described above (U.S. Pat. Nos. 5,479,254 and 5,671,045). One improvement is to swirl the gas flow for improved plasma confinement. A nebulizer was added to provide a controlled amount of an element to the device for its calibration. There are no teachings or suggestions to reduce competing emission for flue gas components or enhancing the metals emission.
The same approach is employed for a portable field unit for measuring metals in gas streams. U.S. Pat. No. 5,825,485 describes the same device as in U.S. Pat. No. 5,479,254 described above, but uses a pulsed microwave power supply to decrease power consumption so that it can operate off of batteries which can lighten and shrink the size of the instrument to make the device portable. Again, there are no explicit provisions for reducing competing emission from flue gas components nor enhancing the metals emission. There are no examples of measurements or mentioning of detection levels.
U.S. Pat. No. 3,843,257 is one of the first uses of a microwave emission detector to analyze metals and non-metallic compositions. It operated at pressures of 1 Torr and less where there was an increase of sensitivity. The microwave power was applied continuously and there were no other provisions to improve instrument sensitivity or reduce competing background emission. There are no examples of measurements or mention of detection levels.
To help measure particles in a gas stream U.S. Pat. No. 5,854,431 describes a screen to collect particles. After collection the screen is heated to release vapors and particles for analysis in a particle or vapor detector, such as an ion mobility detector. The device is a particle pre-concentrator utilizing a screen.
U.S. Pat. No. 5,242,143 improves the measurement of trace constitutes in gases with a pre-concentration apparatus. The apparatus uses a sorbent where trace gases are sampled at high pressure near atmospheric pressure and desorbs in a carrier gas at low pressure and low flow rates. In this case the relative mass of the trace constitutes in the carrier gas is much greater than in the sample gas, a clear benefit for a mass spectrometer benefit.
The approaches used in the art do not deal with the specific means of reducing interfering emission from flue gas, or means of reducing quenching of metals emissions of interest. These two effects prevent sensitive measurement of metals and specifically mercury at concentrations as low as 10 parts per trillion in flue gas.
The present invention measures metals in gas streams by employing plasma emission spectroscopy. Plasma emission spectroscopy by itself, however, is insufficient for measuring concentrations as low as 10 parts per trillion in flue gas because of a number of physical issues. Flue gas consists of an ensemble of gases including nitrogen, oxygen, carbon dioxide, sulfur dioxide, nitric oxide, and nitrous oxide. Very high concentrations of water vapor (18%) are also present.
The plasma source is based upon a resonant-high-intensity reentrant microwave cavity (FIGS. 1a and 1b). The reentrant cavity is cylindrical in shape with a coaxial center conductor connected at one end of the cylinder and on the other end is a small gap. The plasma is formed in the small gap area. The height of the cylindrical part is adjusted to match the resonant condition for a pure coaxial cavity. The size of the gap is adjusted to match the resonant condition for the reentrant cavity. There is a small aperture in the cylinder""s wall for the microwave power to flow into the cavity from the microwave generator. The microwave power flows through a waveguide that is connected to the microwave cavity. There is a quartz window in the waveguide to provide a vacuum seal and allow the transmission of the microwave power from the generator to the plasma source (FIG. 1b). Alternately, the window can also be placed in the cavity in the form of a circular quartz tube mounted through the center conductor and exhaust hole in the cavity (FIG. 1a). In addition, the waveguide can be simply replaced with a coaxial wire feed from the power source. The plasma source can be designed to operate in the range of about 30-10,000 MHz. Measurements are made by pulling a gas stream into the cavity with use of a pump. The gas enters the cavity through the center conductor through a small hole and flows into the plasma region. The gas then exits the cavity through a small hole on the opposite side wall. The instrument can also be built with cylindrical and other microwave cavities.
A first configuration is shown in FIG. 2. A sampling probe is mounted in a flue duct to sample the gas stream. An optional filter may follow the probe to remove particulate matter from the gas stream. A heated sample line delivers the gas stream to the plasma source. A flow meter or a flow restriction before the plasma source regulates the gas flow. The plasma source is pulsed to enhance the trace metal signal compared to plasma emission background. The ultra-violet light emanating from the plasma is coupled to a spectrometer with either a fiber optic cable or a lens. The spectrometer resolves the light intensity from the trace metal line (for example, the mercury line near 253.65 nm) and the background light intensity near the trace metal line. These two light intensities are measured with detectors on the output of the spectrometer. The two light intensities are integrated over many pulses, for example, by box-car averaging techniques. The trace metal light immediately after each plasma pulse is enhanced compared to the background light. The integrated intensities after each pulse are subtracted and is proportional to the trace metal density. The detectors can be, for example, a CCD camera, a photo diode array, or a photomultiplier. The instrument is calibrated by measuring a known amount of trace metal vapor in a gas stream.
An absorbent can be utilized to collect the trace metal of interest and subsequently released for analysis. The absorbent will collect the trace metal and not much of the gas flow. This use of an absorbent is particularly effective in the collection of trace metal, mercury e.g., vapor in gas streams. The trace metal is collected with the absorbent at a pressure below atmosphere (40-300 Torr) to prevent the accumulation and condensation of water vapor near the absorbent. The collection time is less than 2 minutes and analysis time is less than 30 seconds. The trace metal is released by heating the absorbent. The gas flow (0.1-1 ml/min) is changed to either argon or nitrogen gas to deliver the trace metal to the plasma source, and produce a plasma in the source with a small emission background compared to the trace metal signal by reducing the levels of oxygen in the plasma source. In this configuration the absorbent eliminates problems with pressure fluctuations in the plasma source, reduces background emission and quenching problems, and ultimately increases signal-to-noise of the measurement. Furthermore, the absorbent can deal with the problem of large water vapor and acid content in the gas streams. For this configuration the plasma source is either pulsed or operated continuously. Although continuous plasma operation is not as detrimental with this second configuration, pulsed operation still has a significant advantage of reducing the competing emission background. The ultra-violet light emanating from the plasma is coupled to a spectrometer with either a fiber optic cable or a lens. The spectrometer resolves the light intensity from the trace metal line and the background light intensity near the trace metal line. These two light intensities are measured with detectors on the output of the spectrometer. The two intensities are subtracted and it is proportional to the trace metal density. The detectors can be a CCD camera, a photo diode array, or a photomultiplier. The instrument is calibrated by measuring a known amount of trace metal vapor in a gas stream.
A third configuration enables the measurement of metal content on particles in gas streams. The first two instrument configurations measure metals in the vapor phase in gas streams. The typical metals that have been measured were mercury, arsenic, and selenium because they are in the vapor phase for temperatures below 140 degrees C. The pulse operation of the plasma source doesn""t provide sufficient average power to vaporize solid metals or break the bonds of molecular metals. The second configuration is modified (FIG. 4) to measure a larger number of metals that are typically in a solid form below 140 degrees C. (e.g. lead, chromium, magnesium, manganese, and zinc). A particle collection system is added to collect particles in gas streams and heat them to high temperatures (e.g.,  greater than 1500 degrees F.) where they melt, their molecular bonds are broken, and they enter the vapor state. The particle collection system consists of a high temperature ceramic in the shape of a cylinder with a pin-hole in the bottom to allow the heated metals to stream out of the ceramic cylinder. The ceramic cylinder is porous to allow gas to be pulled through the ceramic pores, but trap the particles. The heating is achieved, for example, with 200 watts of AC power supply heating Nichrome wire wrapped around the ceramic cylinder. The particle collection system collects particles by pumping a gas stream loaded with particles through the ceramic cylinder. The gas stream flows into the ceramic cylinder, through the porous ceramic, and out a small pumping duct. The ceramic cylinder captures the particles and passes the gas stream. After collection, a non-porous ceramic shutter closes off the open end of the ceramic cylinder and a flow valve shuts off the gas stream collection. Next a purge gas bottle supplies purge gas to the ceramic cylinder through its porous walls. Then the AC power supply heats the Nichrome wire and, through conduction, heats the ceramic cylinder to temperatures in excess of 1500 degrees F. When the particles in the ceramic cylinder reach these temperatures the metals will melt, boil off, and exit the ceramic cylinder as a vapor and into the plasma source. The ultra-violet light emanating from the plasma is coupled to a spectrometer with a fiber optic cable, a lens, or both. The spectrometer resolves the light intensity from the metal lines and the background light intensity near the metal lines. These two light intensities are measured with detectors on the output of the spectrometer. The two intensities are subtracted and the difference is proportional to the metal density. The detectors can be a CCD camera, a photo diode array, or a photomultiplier. The instrument configuration can be used to analyze fly ash in coal fired utility plants, contaminated soil, or particulate from manufacturing plants.
These and other features of the invention will be more fully understood by reference to the following drawings.